1. Field of the Invention
The present invention relates to image pickup apparatus, for example such as cameras with a function to capture a certain area in a photographed image as video information and to automatically adjust the focus according to the information, particularly provided with means for changing the position of the capturing area.
2. Related Background Art
A variety of ideas have been presented for obtaining higher-quality images more simply in the fields of consumer-oriented image pickup devices including camcorders. The autofocus (AF) function, coming standard in these years, is for eliminating inconvenience of adjusting the focus every shot, which can be said as a typical example of a function to achieve a purpose to obtain good images simply.
Incidentally, because AF is a mechanism in which an image pickup device such as a camera judges photographic circumstances, so to speak, "at its own discretion" and adjusts the lens position into a state that must be suitable for the circumstances, there are not a few cases where an image does not reflect a photographic purpose of a photographer.
For example, if a far object and a near object coexist in a photographic screen and if the AF operation is carried out using information of the entire photographic screen, the image pickup device will become in focus to either one of the plural objects, but the image pickup device will not be able to determine whether it is surely the main object to be focused.
In order to avoid such circumstances as much as possible, it is usual to employ a technique for performing distance measurement with a weight on an object located at the center of photographic screen and executing AF based on a result of the distance measurement. The reason for this is the fact that the photographer often captures a main object in the screen center upon shooting. This technique has a drawback that the focus cannot be properly adjusted to a main object which is located at a place other than the screen center.
The applicant of the present invention has proposed an image pickup apparatus arranged in such a manner that a photographer looking into a finder can select a main object by his visual axis so as to achieve the best focus no matter where the main object is located in the photographic screen, in Japanese Patent Application No. 4-154165.
FIG. 13 is a drawing to illustrate the operation with the image pickup apparatus. In FIG. 13, reference numeral 800 designates the photographic screen, 102 an image of an object, and 801 a focus detection area (distance measurement frame). The focus detection area 801 is set at a position of a photographer's viewpoint, and the setting position thereof can be freely moved, for example from 801 to 802 in the drawing, by the photographer's changing the position of viewpoint.
The position designating means for selecting the main object does not have to be limited to the viewpoint detecting means, but may be another means for determining a moving direction and a moving position by combining moving amounts on two axes with each other, for example such as a joy stick or a mouse.
In the cases as described above, the image pickup apparatus detects the position of the photographer's viewpoint or the position of another position designating device, and then moves the distance measurement area, corresponding to the detected position.
Incidentally, recent AF systems generally employ a method for detecting a signal according to sharpness from a video signal and using it as an AF evaluation value, but one of the objects hard to deal with by such an AF control using the AF evaluation value is a low-contrast object.
With a low-contrast object, a changing state of AF evaluation level against focus lens position is like a waveform 1102 shown in FIG. 11. The level of AF evaluation value rarely changes no matter whether the object is out of focus or in focus, and the level is wholly so low as to be buried in the noise of electric circuits, which will often result in disabling the detection of change.
In such cases, if the lens starts moving toward the in-focus point, because of influence of the noise the lens could move in the opposite direction to the in-focus point or the lens could continue reciprocating as failing to find a maximum value (which is so called as an action of hunting), thereby giving photography a hitch.
If the focus detection area (distance measurement frame) is moved as shown in FIG. 13, the detection area could capture only a low-contrast portion in the screen though the photographic screen is not a low-contrast object as a whole. Thus, there is a possibility to increase chances to cause the above hitch on photography. A specific example thereof is shown in FIGS. 14A and 14B.
FIGS. 14A and 14B show photographic screens where a man is standing in front of a contrastless background.
For example, if the system is provided with a function to detect the position of viewpoint, as the viewpoint is moved from point 1008 to a position of point 1009 as shown in FIG. 14A, the focus detection area also moves from 1001 to 1002 in the drawing.
Since the focus detection area 1001 includes an image of the man, the man's image can be brought into focus by carrying out the AF operation in this state.
However, the detection area 1002 does not include the man and captures only a portion with no contrast at all as video information. When the AF operation is executed in this state, it will result in moving the focus lens 103 to cause the above-mentioned hitch on photography.
FIG. 14B shows an example in which character indications indicating operational conditions of a camera etc. are superimposed on the display screen of a view finder (a monitor screen of LCD display circuit 109 as detailed later).
In FIG. 14B, numeral 1003 denotes an indication of a type of recording tape (or a recording method) and 1004 indicates that the camera is recording.
Also, 1007 is an indication indicating that information of the date etc. is superimposed on an image to be recorded. The photographer performs photography as always checking such indications through the eye.
For example, if the photographer checks the date after looking at the position of 1010 where the man in the focus detection area 1006 is in focus, the position of the viewpoint moves to 1011 and the focus detection area also moves to 1007.
Although the photographer would like to continue focusing on the man, the focus lens unnecessarily moves also in such a case because the area inside 1007 has no contrast at all as a photographic image as in 1002. This will result in causing unsharpness of the image.